Electric discharge devices



Dec. 20, 1960 H. DE BOYNE KNIGHT 5, 8

ELECTRIC DISCHARGE DEVICES 2 Sheets-Sheet 1 Filed Nov. 30, 1959 conducting d lnsularlng /2{ materials conduchn I lnsulahng materials //V V! N TOR HENRY DE 5 If/V/GHT 47'7'0FA EY Dec. 20, 1960 H. DE BOYNE KNIGHT 2,965,788

ELECTRIC DISCHARGE DEVICES 2Sheets-Sheet 2 Filed Nov. 30, 1959 N/GHT HENRY DE 50 ATTORNEY United States Patent ELECTRIC DISCHARGE DEVICES Henry de Boyne Knight, Rugby, England, assignor to The British lhomson-Houston Company Limited, London, England, a company of Great Britain Filed Nov. 30, 1959, Ser. No. 856,280 Claims priority, application Great Britain Dec. 2, 1958 8 Claims. (Cl. 313-193) This invention relates to electric discharge devices of the kind in which the discharge has the characteristic of an arc, and occurs between an electron-emitting electrode and an anode in an envelope which contains an ionizable medium at a pressure, under operating conditions, which is generally not greater than two thousand microns of mercury.

1n the specification of application Serial No. 839,937 filed by me on 14th September 1939, and assigned to the assignee of the present application, devices are described in which a low-pressure gas or vapour discharge is initiated on the path between an anode and an auxiliary thermionically-emitting cathode and transfers to a path between the anode and an are spot which forms on the main cathode, from which emission is of the cold cathode type. The transfer is encouraged by causing the initial discharge to pass through a restricting aperture in or adjacent to the main cathode and providing insulating members or particles in close relation to conducting members or particles on the main cathode adjacent to the aperture.

The initial discharge may be preceded by a priming discharge from an auxiliary anode, or from a grid member which may be interposed in the initial discharge path for the purpose of controlling the initiation of the discharge.

In the device according to the invention, the main cathode is arranged to separate the discharge space between the anode and the main cathode so that the pressure therein is maintained during operation at a pressure lower than that in the space between the main cathode and the auxiliary cathode.

By this means, the relatively lower perssure in the anode/main cathode space favours the possibility of applying high voltages across this space without the occurrence of an electrical breakdown until this is initiated by the initial discharge to the auxiliary thermionically-emitting cathode. At the same time, the relatively higher pressure in the main cathode/ auxiliary cathode space favours the easy establishment of the initial discharge and a limitation of the effect of ion bombardment on the auxiliary cathode. The relatively low pressure in the anode/main cathode space also favours rapid deionisation of the space after current conduction stops. Perforated baffies may be included in this space, if desired, to be used for voltage division, for control, or for assisting deionisation. The pressure of gas or vapour in this space may be initiallytoo low to support the main arc current, but may be augmented by gas and/or vapour emitted from the main cathode by the action of the arc spot.

In order to obtain the desired differential, the discharge conducting medium consists of a condensible vapour, such as that of mercury or caesium, and the interior of the device is separated into two effective chambers, corresponding to the two interelectrode spaces described above, communication between the two chambers being by way of one or more passages which present a high impedance to vapour flow, but suffice to allow the passage of ionised particles associated with the initial discharge. The effective vapour pressure in each chamber is determined by the temperature of the coldest spot on walls or electrodes towards which vapour can treely flow. in carrying out the invention, the cold-spot temperature in the anode/main cathode space is caused to be substantially lower than that in the main cathode/ auxiliary cathode space.

rigs. l, 2 and 3 of the accompanying drawings show in cross section, three illustrative examples of embodiments of the invention.

in Fig. l, the device comprises a vacuum-tight envelope 1 into which are sealed, with connecting leads passing through the envelope wall, an anode 2, an apertured main cathode 3, a perforated control electrode 4 and a thermionically-emitting auxiliary cathode 5, shown as a directlyheated type, which can be heated to emitting temperature by the supply of current through the appropriate leads. After processing and evacuation the envelope is sealed off at 6, a quantity 7 of vapourisable material, in this case mercury, being included to provide the dischargeconducting vapour.

In operation, a discharge is initiated between anode 2 and auxiliary cathode 5, with the co-operation of electrode 4, and passes through the ap-rture in main cathode 3, thereafter transferring to arc spots on the surface of the main cathode initially in the vicinity of the aperture therein. To encourage the formation of such spots the main cathode 3 may comprise a mass formed of a sintered mixture of metal and insulating particles, or an insulating member in close association with a conducting member such as nickel, graphite or aluminium, may be located in the neighbourhood of the aperture.

In accordance with the present invention, the main cathode 3 is constructed so as effectively to divide the space within the envelope into two compartments between which there can exist a substantial difference in vapour pressure.

In the example illustrated in Fig. 1, this efiect is realised by means of a frusto-conical member 8 of sheet metal, fitting close to cathode 3 on its inner diameter and having a cylindrical extension 9 which extends close to the inner wall of the envelope; the annular spacing between exten sion 9 and the inner wall of envelope 1 provides an adequate impedance to flow of vapour from the higher to the lower pressure Zone. During operation, there may, for example, be mercury vapour at a pressure of one micron or less in the anode/main cathode chamber, and at a pressure of 20 microns Hg or more in the main cathode/auxiliary cathode chamber. Member 9 also serves to impede the passage of ionised particles between the anode 1 and the auxiliary cathode 5, so as to restrict the initial discharge to the path through the aperture in 3. Other restrictive members 10, 11 of cylindrical form may be added for the same purpose. Member 10 may extend close to the end of envelope 1, still further restricting the higher pressure zone. Cylinders 10 and 11 also serve as heat shields to restrict the loss of heat by radiation from the auxiliary cathode 5. Extra heat-shielding members may be added if desired.

During operation, the heat dissipated by 5 raises the cold-spot temperature in the space bounded by 3, 10 and the lower end of the envelope, to a value corresponding to the vapour pressure desired; if necessary heat may be applied to the lower end of the envelope to help the attainment of the required temperature. Some portion of the envelope forming the boundary of the lower pressure chamber, for example over an area around the envelope at the level 12, is maintained at a temperature corresponding to the lower pressure desired. Cooling may, for example, be by forced air.

Under these conditions there is a constant flow of vapour from the hi her pressure to the lower pressure zone, through the aperture in 3 and through the annular Patented Dec. 20, 1960 space between 1 and 9. This vapour is condensed on the cooled section 12 and the liquid returns to the higher pressure zone via the annular space. The latter space should be made as small as possible, consistent with manufacture; it may be necessary to provide a special opening inmember 8 for the return of the condensed mercury.

Fig. 2 shows an alternative construction in which the separation of the envelope into two chambers is eltected by hermetically sealing a dividing member to the envelope. The anode is shown as a sheet metal pressing 13, which may carry a facing member 17 if desired. The main cathode member 3 is carriedby a second sheet metal pressing 14. Members 13, 14 are provided with radial flanges which are hermetically sealed to a glass envelope cylinder 15. A glass envelope cylinder 16 sealed to the other face of 14 also carries a b:se memher which supports electrodes 5 and 11 and through which the connecting leads to those electrodes pass. Connection may be made to electrode 3 by means of the portion of member 14 extending beyond the envelope. The members 13, 14 may be hermetically sealed to memb'ers 15 16 by known techniques. For example 13, 14 may be of an iron-nickel-cobalt alloy of the type known under the registered trademark Kovar, and 15, 16 may be a boro-silicate glass having an expansion characteristic matching this metal. In an alternative construction members 15, 16 may be of ceramic material, 13 and 14 being of suitable metal sealed by a known process (for example the titanium hydride process). In the latter case the base supporting electrodes 5, 11shown in Fig. 2 as an extension of member 16-might be a separate metal member sealed to 16 and carrying the connectingleads through insulation inserts. The pressure within envelope 15 may be reduced by cooling it at level 18. Condensed liquid returns to the base via hole .1? in m mber 14.

" In Figs. 1 and 2 the envelopes are shown as sealed-oft after evacuation at 6. If desired they may be connected yia 6 to means for continuous removal of gas, for example gas evolved by the action of the arc spots on members 3, 8, 14. Also, for the removal of such gas, means may be provided within the envelope, such as a quantity of getter material, for example metal titanium or zirconium, these being maintained at a temperature at which they are effective in cleaning-up the undesired gas.

In the embodiment of the invention illustrated in Fig. 3,'the envelope of the device consists mainly of metal. The cylindrical metal envelope 31 is closed at the ends by metal headers 32, 33, sealed in vacuum tight manner. An anode34, afiixed to an anode lead 35, is supported by the header 32 and is insulated from the header by a glass sleeve 36 which is hermetically sealed both to the header 32 andto the anode lead 35 through the intermediary of members 37 and 38 of suitable metal. A stress shield 39 is interposedbetween lead 35 and the junction of headers 32 with member 37. An evacuating tubulation is shown at 40. A conducting lead 41 in contact with member 38 serves as a terminal for the anode.

A mercury-pool type cathode is employed. A semiconductor type ignitor 42 and an auxiliary anode 43 (if required) are supported upon leads passing through the header 33 and insulated therefrom by glass-to-metal seals 42', 43, respectively. ignitor 42 dips into the mercurypool 44. A metal lead 45 attached to header 33 serves as cathode terminal.

Between the anode 34 and the ignitor 42 is located a baffle 46 of conducting material which together with part of envelope 31 serves as the main cathode of the device. Battle 46, which has one or more apertures 47 for the passage of the arc'discha'rge is preferably so shaped that mercury condensing above it will tend to run towards aperture 47 or towards an opening 48 from which it returns to the pool 44 via a tubular member 49. Member49 may or may not dip into pool 44,, b t is S0 ar- 4 ranged as to prevent undue escape of vapour through it into the space above 46.

In operation, the device is""connected into a main circuit through terminals 41, 45; current flow is initiated therein by the passage of a current pulse through the ignitor 42 and the mercury pool 44; a cathode spot being formed at the point of immersion, and ionised particles being set free in the space. Provided the degree of ionisation is sufficient and that the anode 34 is at a suitably high positive potential a discharge passes from anode 34 to the cathode spot through the aperture 47. If desired, the ionisation may be increased by the formation of an arc discharge from the auxiliary anode 43 to the mercury pool cathode 44 in a separate circuit. If the current density in the aperture 47 and the anode potential are high enough, the discharge between anode 34 and pool 44 transfers to a path from anode 34 to cathode spots formed on baffle '46, initially in the vicinity of the aperture 47, and further spots may form parts of the bafile 46 remote from aperture 47 and also on the envelope 31. The surface of battle 46 and/or the inner surface of 31 may, if desired, be supplied with means for encouraging the formation of cathode spots by the inclusion of member 50, which is a perforated disc of insulating material, preferably ceramic, such as sintered alumina, held in close contact with the member 46 and surrounding the aperture 47.

It will be seen that the bafile 46 divides the device into an upper and a lower chamber. The vapour pressure in the upper chamber may be kept as low as des red by cooling a portion of the envelope (for example by means of a water jacket 51 with inlet and outlet ports 52, 53), and/or the battle member 46 (by water cooling means not shown). For example in the case of mercury vapour, the pressureis limited to about 0.3 micron of mercury if a substantial area of the envelope is kept at 5 C. The vapour pressure in the lower chamber may be allowed to be higher than that in the upper chamber; the mercury diffusing as vapour through aperture 47 condenses on the walls and is returned via opening 48 and tubular member 49 to the lower chamber. If the vapour blast associated with the initial cathode spot formed on the pool 44 is not enough to ensure rapid formation of the arc to the anode 34, the pressure in the lower chamber may be brought to any desired level, for example by the supply of heat to the pool 44, if necessary.

When an arc is initiated to the cathode 44, the resulting vapour blast, if any, will tend to increase the vapour pressure in the space below bafiie 46. However, as a result of the small cross-section of the aperture 47, the increase in pressure is not communicated to any substantial degree, to the space above the bafile. The pressure in the space above the bafile thus remains below that present in the space below the battle; thus maintaining the insulation between the anode and the baffie. If water jacketing is resorted to as above mentioned, the vapour pressure in the space above the bafile may be controlled by controlling the temperature of the water.

It is desirable that the surface of insulating sleeve 36 should be free from condensed mercury. For this reason it iscommon practice to heat the sleeve and the neighbouring parts of the envelope, for example by radiant heat, to remove any condensate and prevent further condensation during operation. Any liquid mercury collecting at the bottom of the stress-relieving member 39 is allowed to escape through holes such as indicated at 54.

What I claim is;

.1. An electric arc discharge device comprising a sealed envelope containing an anode, and an electron-emitting cathode, saidenvelope being filled with an ionizable medium at a pressure such that a discharge between said anode and said cathode is of an arc-like character, electrodemeans located-in the discharge space between said cathode and said anode, said electrode means having an aperture therein for the] passage of said discharge and being located so as efiectively to separate said envelope into a discharge space containing said anode and a discharge space containing said cathode, insulating material and electrically conducting material in contact with each other and located in the vicinity of said aperture, said insulating material possessing the ability to encourage the formation on said electrically conducting material of the spot to which said are transfers from said cathode.

2. An electric arc discharge device comprising a sealed envelope containing an anode, and an electron-emitting cathode, said envelope being filled with an ionizable medium at a pressure such that a discharge between said anode and said cathode is of an arc-like character, electrode means located between said cathode and said anode, said electrode means having an aperture therein for the passage of said discharge, and being located so as effectively to separate said envelope into a discharge space containing said anode and a discharge space containing said cathode, and insulating material surrounding said aperture and in contact with said electrode means to encourage the formation on said electrode means of a spot to which said are transfers from said cathode.

3. An electric arc discharge device comprising a sealed envelope containing an anode and a cathode of vaporizable material adapted to produce, during operation, an ionizable vapour for supporting said are discharge, electrode means located in the path of a discharge between said cathode and said anode, said electrode means having an aperture therein for the passage of said discharge and being located so as eifectively to separate said envelope into a discharge space containing said anode and a discharge space containing said cathode, and insulating material and electrically conducting material in contact with each other and located in the vicinity of said aperture, said insulating material possessing the ability to encourage the formation on said electrically conducting material of a spot to which said are transfers from said cathode and means for initiating a discharge from said anode to said vaporisable cathode.

4. An electric arc discharge device as claimed in claim 3, having an auxiliary anode located in the discharge space containing said vaporizable cathode, said auxiliary anode being located adjacent the path of a discharge through said aperture.

5. An electric arc discharge device as claimed in claim 3, having means for cooling the discharge space 45 2,802,131

containing said anode to assist in the reduction of vapour pressure in said space.

6. An electric arc discharge device according to claim 5, having means for returning to said cathode vaporizable material condensed in the discharge space containing said anode.

7. An electric arc discharge device comprising a metal envelope, an anode insulated from said envelope, a cathode of vaporizable material in contact with said envelope, an electrode associated with said cathode for initiating an arc discharge between said anode and said cathode, and electrode means located so as to separate said envelope into a discharge space containing said cathode and a discharge space containing said anode, said electrode means having an aperture therein for the passage of a discharge between said cathode and said anode, and insulating material surrounding said aperture and in contact with said electrode means to encourage the formation on said electrode means of a spot to which said discharge transfers from said cathode.

8. An electric arc discharge device comprising a sealed envelope containing an anode, and an auxiliary electronemitting cathode, said envelope being filled with a condensible ionizable medium at a pressure such that a discharge between said anode and said auxiliary cathode is of an arc-like character, a main cathode located in the discharge space between said cathode and said anode, said main cathode having an aperture therein for the passage of said discharge, and being located so as efiectively to separate said envelope into a discharge space containing said anode and a discharge :space containing said auxiliary cathode, a discharge control electrode located between said auxiliary and main cathode, and insulating material in contact with electrically conducting material located in the vicinity of said aperture, said insulating material possessing the ability to encourage the formation on said electrically conducting material of a spot to which said are transfers from said auxiliary cathode to said main cathode.

References Cited in the file of this patent UNITED STATES PATENTS Meyer et a1. Aug. 6, 1957 

